In　order　to　solve　corrosion　problem　of　soldered　joint　when　fluxing　agents　were　used,　a　technology　of　ultrasonic　soldering　was　developed.　A　lap　joint　of　magnesium　alloy　AZ31B　was　obtained　successfully　in　air　with　ultrasonic　aided.　The　microstructure　and　strength　of　the　joint　soldered　by　this　method　were　studied.　The　effect　of　ultrasonic　vibration　time　and　width　of　clearance　on　shear　strength　of　joint　was　investigated.　The　results　indicated　that　ultrasonic　vibration　can　remove　the　oxide　layer　on　base　metal.　And　the　shear　strength　of　joint　reaches　80-90　MPa　in　the　appropriate　range　of　vibration　time.　The　shear　strength　decreases　when　the　width　of　clearance　is　too　large　or　too　small.　The　microstructures　of　joint　are　mainly　α-Mg　and　Mg-Zn　phases　and　the　fracture　mode　is　intergranular　fracture.